This invention relates generally to spread spectrum communication systems that utilize an inter-frequency search. More particularly, a technique for performing an inter-frequency search with reduced or eliminated loss of link frames is described.
Wireless communication systems have grown dramatically in popularity in recent years. In typical wireless communication systems, mobile stations (e.g., a cellular telephone) communicate with other mobile stations via base stations. To date, a variety of cellular networks have been implemented, and one of the increasingly popular types of networks is referred to as a code division multiple access (CDMA) system.
FIG. 1 provides an overview of a CDMA system. In this system, mobile switching center 10 provides simultaneous communications among multiple base stations 20 while simultaneously routing calls from one or more base station 20 to public switched telephone network (PSTN) 30. PSTN 30 communicates with, for example, telephone 40. The mobile switching center's simultaneous routing makes handoffs between base station 20 and other base stations more reliable. CDMA base stations use one or more CDMA radio channels to provide both control and voice functionality. Base station 20 converts the radio channel to a signal that is transferred to and from mobile switching center 10. Base station 20 can also communicate simultaneously among different sections in a cell, enhancing handoffs. Base station 20 communicates with, for example, mobile stations 50, 52 and 54. Mobile stations 50, 52 and 54 can be, for example, mobile telephones and other types devices that provide wireless communication, such as PCSs, laptop computers or PDAs.
In a CDMA system, there are two types of handoffs, soft and hard. In a soft handoff, the mobile station is allowed to communicate with two or more cell sites enhancing the signal quality. These cell sites must share the same frequency. The CDMA mobile station measures the pilot channel signal strength from adjacent cells and transmits the measurements to the serving base station. The pilot channel provides a reference for coherent channel demodulation and is used as a reference signal level for handoff decisions. The mobile station must be synchronized to the pilot channel pseudo noise (PN) phase before accessing any other control channel. When an adjacent base station's pilot channel signal is strong enough, the mobile station moves the base station pilot into a candidate set and sends a pilot strength measurement message indicating the pilot signal energy. Now, both base stations (i.e., the current and the new one) send an extended handoff direction message, which requests the addition of the new base station pilot to the mobile stations active set of pilots. The new base station also starts transmitting a signal to the mobile station, while the mobile station tunes to the arriving signal from the new base station. This tuning occurs when the mobile station assigns a demodulating element (e.g., a finger on a rake receiver) to the arriving signal. Thus, during the soft handoff, the mobile station is communicating with both base stations simultaneously. During soft handoff, the mobile station utilizes time diversity to use signals from both base stations. The mobile station adds the new signal in a maximum ratio combiner before the decoding.
In contrast, during a hard handoff, the mobile station terminates the communication link with the current servicing base station before establishing the link with the new base station. This technique is similar to the technique used in time division multiple access (TDMA) and global system for mobile communication (GSM) systems. Hard handoffs occur when the mobile station's receiver is switching between a base station of one frequency and a base station of a different frequency. Usually, there is only one receiver in a mobile station, and that receiver can only receive data from one frequency at any given time. Therefore, a soft handoff is not possible when switching between base stations with different frequencies. FIG. 2 illustrates base stations with different frequencies. For example, cells 100, 104, 108, 114, 116 and 118 use a first frequency and cells 102, 106, 110, 112 and 120 use a second frequency. Microcells 106 and 110 are used, for example, in shopping malls, office buildings and other indoor facilities.
Currently available hard handoff techniques can result in a dropped or lost telephone call. If the searcher in the cellular telephone mobile station uses, for example, a sequential sliding correlator (SSC) algorithm, and the search window size is 192 clips, then the total search time for a typical system with a 1× spreading note is as follows:                     SF        =                ⁢                  192          ×                      C            L                    ×                                    0.8                              -                6                                      ⁡                          [              S              ]                                                              =                ⁢                  192          ×          768          ×                      0.8                                          -                6                            =                                ⁢          0.18          ⁢                                           ⁢          seconds                    where: CL is the average correlation length to achieve 0.99 depiction probability or approximately 20 frames. Because the frequency search message contains more than one base station offset in this example, the loss of service quality can be significant if the mobile station performs all searches in one period. If 20 frames are lost, then the telephone call will likely be so dropped.
The IS-95 standard combines new digital CDMA and advanced mobile phone service (AMPS) functionality. IS-95A CDMA systems do not allow for inter-frequency searches because of the continuous nature of the CDMA waveforms. The inter-frequency search, also called mobile assisted hard handoff, was introduced in the IS-95B CDMA standard. The mobile assisted hard handoff can be performed without any timing restrictions (i.e., there is no restriction on the length of time used for this handoff). As a result, the mobile station is allowed to erase as many data frames (or portions of data frames) of the forward or reverse links as needed to perform the inter-frequency search.
The forward link is the data link from the base station to the mobile station, and the reverse link is the data link from the mobile station to the base station.
Currently, the CDMA mobile station performing the inter-frequency search will erase one or more of forward link frames and reverse link frames.
The IS-95B standard includes a gated-off transmission technique on the reverse link. A gated-off transmission is used when voice activity is low, and this allows voice data to be sent at different rates depending on the voice activity. For example, when voice activity drops to a low rate (e.g., ⅛ of the full rate), the transmission can be gated-off such that ⅛ of the normal amount of date is transmitted. This gated-off transmission on the reverse link allows the mobile station to perform the inter-frequency search during the period when the transmitter is gated-off. This technique minimizes the impact on the reverse link.
A mobile station in a third generation CDMA system, such as cdma2000, does not gate-off its transmitter during the transmission of lower rates. Thus, the erasure (i.e., loss) of both forward and reverse link frames is particularly true in this situation. Most of the third generation CDMA systems allow for a mixture of different classes of service, such as speech over data. In the cdma2000 system, this is achieved by allowing a simultaneous transmission on many physical channels. For example, fundamental channels and supplemental channels each carry a different payload. Additionally, since speech is carried on a fundamental channel and uses variable (i.e., speech activity driven) data rates, while a supplemental channel usually uses higher fixed (i.e., assigned) data rates, it is eminent that one or both of these channels will experience erasure during the inter-frequency search. Therefore, the likelihood of erasing links increases with, for example, the cdma2000 standard because the cdma2000 standard allows the mixing of different classes of services (e.g., data and voice services) on the fundamental and supplemental channels. It is desirable to have an inter-frequency search with minimal loss link frames in a CDMA system.